Recently, compact digital camera and digital video camera using a solid-state image sensor, such as a CCD and a CMOS, have been integrated into a mobile phone or a personal digital assistant. A surveillance camera having a built-in digital video camera has been getting smaller in size. These cameras need a miniature optical system suitable for a miniature solid-state image sensor, and it is convenient when the miniature optical system is, preferably, a zoom lens of a structure such that can vary a focal length of the overall lens system continuously while keeping the image-forming position on the same plane.
As a miniature zoom lens of a simple configuration using fewer lenses, a three-component zoom lens has been known, which comprises, sequentially from the object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power as has been disclosed, for example, in the following documents, referred to hereafter as Patent Documents 1 through 4:    Patent Document 1 (JP-A-5-323190);    Patent Document 2 (JP-A-11-237549);    Patent Document 3 (JP-A-2002-55278); and    Patent Document 4 (JP-A-2003-177315).
Each of the three-component zoom lenses in the related art, however, has problems. For example, the zoom lens disclosed in Patent Document 1 is of a simple configuration by forming each lens group from a single lens; however, it fails to attain a zoom ratio of two times and the performance as a zoom lens is therefore insufficient. The zoom lens disclosed in Patent Document 2 is for use in an electron still camera, and an interval between the first lens group and the second lens group is 13 mm or longer. Hence, this zoom lens is not small enough to be incorporated into a mobile phone or the like. The zoom lens disclosed in Patent Document 3 is of a relatively miniature size by forming each lens group from a single lens and attains a large zoom ratio; however, in order to attain a large zoom ratio, the first lens group and the second group have to move, which causes a total length to vary. This raises a problem that it is difficult to incorporate this zoom lens into a portable device. The zoom lens disclosed in Patent Document 4 is of a relatively miniature size by forming each of the first lens group and the third lens group from a single lens and the second lens group from two positive and negative lenses, and attains a large zoom ratio; however, because the first lens and the second lens have to move, this zoom lens also has a problem that a total length varies.